Desulfurization and decarburization of iron and iron alloys



Patented Jan. 12, 1954 DESULFURZATION AND DECARBURIZA- TION F IRON AND IRON ALLOYS Francis T. Crego, East Orange, and Harold R.

Fisher, Metuchen, N. J., assignors to Air Reduction Company, Incorporated, New York, N. Y., a corporation of New York Application May 5, 1949, Serial No. 91,574

This invention relates to desulphurzation of molten iron and iron alloys such as steel, and particularly to the elimination of sulphur in the open hearth process for the production of steel. The invention also relates to the decarburization of molten iron and iron alloys in the open hearth process after the desulphurization thereof. The invention may be applied to the treatment of any molten bath of iron or iron alloys.

Sulphur is a deleterious element which is normally present in iron and steel manufactured by conventional methods, and at least a portion of the sulphur must be removed to produce metal meeting standard specications and suitable for commercial applications.

The present practice for reducing the sulphur content of iron in the manufacture of steel involves simultaneous removal of carbon. A large excess of lime is added to the slag which, by contact with the metal, causes the sulphur to combine with the lime until an equilibrium condition is established. Decarburization is effected bythe addition of iron ore which oxidizes the carbon in the metal. Oxidation does not, however, promote desulphurization of metals. Rather it tends to produce a reversal of the reaction of sulphur and calcium. Reduction of the sulphur content of the metal is, therefore, relatively slow and especially so when the sulphur content is high. It may require from one to three hours to effect suiiicient desulphurization of .a seventy-ton heat.

It is the object of the present invention to afford a rapid, effective and economical method of eliminating sulphur, or both sulphur and carbon, from molten iron and steel, thereby permitting the attainment of the desired composition in respect to sulphur or sulphur and carbon content in the minimum time.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification and the accompanying drawing, in which the ligure represents a cross-section of an open hearth furnace illustrating the application of the invention to the operation of such a furnace.

The invention depends upon the correlation of certain factors including basicity and fluidity of the slag and the agitation of the bath by means of a gas introduced.

We have discovered that the lime to silica ratio, during the desulphurization, must be at least 2.5:1 and the slag must have a high fluidity which is obtained by adding uorspar in sufcient quantity. Thus, from 50'0 to 600 pounds of iluorspar must be added to the slag employed in a 'T0-ton basic open hearth steel furnace to properly effect the result. Oxidizing agents are eliminated during the desulphurization operation, since oxidation causes a reversal of the reaction between calcium and sulphur and the return of sulphur to the metal and retards the desulphurizing effect of the calcium present in the slag. Thus no iron ore or other oxidizing agent is employed during the desulphurizing procedure.

To elect the necessary agitation during the desulphurization, we introduce an inert gas such as nitrogen through a suitable pipe, for example an ordinary iron pipe which is inserted through the door of the furnace and through the slag layer into the molten metal. Any inert gas free from oxidizing contituents may be used, such as argon and helium, though these gases are probably too expensive for such use in view of the availability of nitrogen.

A reducing gas can be substituted for nitrocompared with an hour to three hours by procedures heretofore known. The time is based upon a -ton heat in an open hearth furnace and mayLvary depending upon the type of apparatus used and the quantity of metal involved.

After the desulphurization has proceeded to the desired point, the slag is removed and a new slag is substituted. Decarburization may then be accomplished either by the conventional method of adding iron ore or by the introduction of oxygen through a -suitable pipe as in the nost modern and eifective decarburizing metho s.

ln the accompanying drawing, we have illustrated an open hearth furnace, although it is to be understood that the invention is not limited to such a furnace, since it may be carried out in any suitable type of metallurgical furnace in which heat may be supplied by combustion, electric arc or otherwise in order to maintain the temperature of the metal.

Referring to the drawing, 5 indicates an open hearth furnace having a roof 6, a hearth 1 and 3 Walls 8, one of the Walls having a charging opening 9. It will be understood that the hearth is fired by the combustion of fuel in the conventional manner, although, as indicated, other heating means may be employed. The bath of molten metal vI on the hearth` is covered by a the proper ratio with the addition of sufiicientL iluorspar to afford the necessary fluidity. `A pipe I2 is inserted through the opening 9 through the slag layer II and into the bath `of molten metal I0, and a suitable gas, such` as nitrogen, other inert gas, a reducing gas or a mixtureof nitrogen and a reducing gasisV delivered under suitable pressure through a flexible connection I3 to the pipe I2 and is thus delivered Vbeneath the surface of the molten metal batti IE) 'to effect violent agitation of the metal. This ensures the most rapid reaction of the sulphur present in the molten metal with the calcium in the slag. Desulphurization proceeds in the manner described until the sulphur content of the metal has reached the desired minimum. Thereafter decarburization may be eiected bythe addition of ironore or by the introduction of oxygenl into the body f the moltenmetal through the pipe I2. In the latter case, the oxygen rapidly burns out the carbon and reduces the carbon content to the desired minimum, while at the same time increasing the temperature of the molten metal.

The procedure as Vdescribed is adapted to effect material savings in the production of iron and steel, especially through the more rapid transfer ,of the sulphur from the metal to the slag, since the reaction is not complicated by the presence'of an oxidizing agent. The saving in time is important, since it increases` the capacity of the equipment of a particular size, thus making the production of metal of low sulphur content more economical. The invention is particularly eiiective with iron containing relatively high proportions .of sulphur Vas compared with the better grades of iron in which the sulphur content is low.

Various Vchanges may be made in the procedure as described as Well asin the apparatus employed without departing from 'the invention or sacrincing the advantages thereof.

We claim:

l; The .method of removing sulphur and carbon from iron or steel which comprises maintaining a molten bath of the iron or steel on an open hearth furnace, maintaining a highly basic andui'd slag having a lime to silica ratio of atleast 2.5'to 1 on the molten bath, introducing a non-oxidizing gaseous medium beneath the sur; face of the molten bath to effect agitation thereof, continuing such introduction of the non-oxidizing gas until a predetermined degree of desulphurization of the molten bath has been obtained, maintaining the molten metal free of oxidizing agents during the introduction of the non-oxidizing gaseous medium, removing the highly basic slag from the molten metal, and then decarburizing the molten bath on the same hearth in which desulphurization was obtained by introducing oxygen into the molten metal bath to combine With carbon of the molten metal and to raise the temperature of the bath.

2. n modified open hearth furnace process for removing sulphur vand carbon from iron or steel which comprises maintaining a molten bath of the iron or steel'in said furnace, maintaining a highly basic and fluid slag having a lime to silica ratio of at least 2.5 to 1 on the molten bath, introducing a non-oxidizing gaseous medium beneath the surface of the molten bath to effect agitation thereof, continuing such introduction of the non-oxidizing gas to change the oxidizing condition of a normal open hearth process to a neutral or slightly reducing condition until a predetermined degree of desulphurization of the molten bath has been obtained, maintaining the molten metal free of oxidizing agents during the introduction of the non-oxidizing gas, removing the highly basic slag from the molten metal, and then decarburizing the molten bath -on the same hearth in which desulphurization was obtained by introducing oxygen into the molten bath to combine with the carbon of the molten metal. Y i

FRANCIS T. CREGQ HAROLD R. FISHER.

Refer-ences cites in the me of this patent UNITED sTATEs PATENTS Number Name Date 1,034,786 Greene Aug. 6, 1912 2,110,066 1-Ie`ue1` Mar. '1, 1938 2,290,961 Heuer July 28, 1942 OTHER REFERENCES 

1. THE METHOD OF REMOVING SULPHUR AND CARBON FROM IRON OR STEEL WHICH COMPRISES MAINTAINING A MOLTEN BATH OF THE IRON OR STEEL ON AN OPEN HEARTH FURNACE, MAINTAINING A HIGHLY BASIC AND FLUID SLAG HAVING A LIME TO SILICA RATIO OF AT LEAST 2.5 TO 1 ON THE MOLTEN BATH, INTRODUCING A NON-OXIDIZING GASEOUS MEDIUM BENEATH THE SURFACE OF THE MOLTEN BATH TO EFFECT AGITATION THEREOF, CONTINUING SUCH INTRODUCTION OF THE NON-OXIDIZING GAS UNTIL A PREDETERMIND DEGREE OF DESULPHURIZATION OF THE MOLTEN BATH HAS BEEN OBTAINED, MAINTAINING THE MOLTEN METAL FREE OF OXIDIZING AGENTS DURING THE INTRODUCTION OF THE NON-OXIDIZING GASEOUS MEDIUM, REMOVING THE HIGHLY BASIC SLAG FROM THE MOLTEN METAL, AND THEN DECARBURIZING THE MOLTEN BATH ON THE SAME HEARTH IN WHICH DESULPHURIZATION WAS OBTAINED BY INTRODUCING OXYGEN INTO THE MOLTEN METAL BATH TO COMBINE WITH CARBON OF THE MOLTEN METAL AND TO RAISE THE TEMPERATURE OF THE BATH. 